The present invention relates generally to semiconductor laser devices and, more particularly, to a high-power ridge laser adapted for coupling to a single-mode optical fiber without additional corrective optics.
With the growth of optical communications, semiconductor lasers have become important components in telecommunication systems. These laser devices enable a high quality of light emission, in particular single transverse mode emission, at relatively high power levels. High power single-mode semiconductor lasers can be used, in particular, as pump lasers for optical fiber amplifiers.
In a standard semiconductor laser, an active region is embedded in a p-n junction. A multilayer structure creates a high refractive index region at both sides of the active region. In such a way, light propagating parallel to the layers can be guided in the active region.
Ridge semiconductor lasers are known that have a region of increased vertical thickness compared with regions lateral to it in the laser. By changing the thickness of layers (for example by selective etching), lateral modulation of the refractive index can be obtained, in order to achieve a light-guiding section. The region with higher thickness (usually called the ridge) has a higher effective refractive index than the lateral regions. The extent of the refractive index step depends on the thickness of the ridge with respect to the lateral regions. Because the real part of the refractive index is higher at the ridge than outside it, light can be guided along the ridge. This guiding mechanism is referred to as index guiding.
As well, current may be injected through metallic contacts deposited on the p and n sides and into the active region. Since the optical gain increases as a function of the carrier concentration, gain is higher in the region beneath the contacts than in the outside region, and laser light will propagate in the high gain region. This guiding mechanism is referred to as gain guiding. In ridge lasers, both index and gain guiding are used as guiding mechanisms, the relative weight of each mechanism depending on the real refractive index change induced by the ridge and on current injection.
In stripe semiconductor lasers, only gain-guiding takes place in the light-guiding section. Stripe lasers are devices in which the injection of charge carriers across one or more semiconductor junctions results in stimulated emission. Mirrored surfaces on the device form a cavity in which the stimulated emission will produce lasing when the injected current density is above a certain threshold level.
For effective use in optical communication systems, for example as pump sources of erbium doped optical fiber amplifiers, semiconductor laser diodes should be able to be coupled efficiently to a single-mode optical fiber, which will carry the light emitted by the laser. Conventional lasers are astigmatic and require corrective optics for compensating vertical divergence of the emitted light, in order to efficiently coupling with the single-mode fiber. Moreover, high-power laser require additional corrective optics in order to achieve an efficient fiber coupling also in the transverse direction. In the following, we will refer to this additional corrective optics simply as xe2x80x9cadditional opticsxe2x80x9d.
Patents and publications disclose various arrangements for gain-guiding and index-guiding in semiconductor laser elements. For example, U.S. Pat. No. 4,251,780 discloses an injection laser of the multilayer planar type having stripe offset geometry on the planar surface of the laser. The patent discloses that the offset geometry is a stripe or substrate channel that is non-orthogonal to the cleaved end facets and stabilizes the transverse mode. A parabolic-shaped or trapezoidal-shaped geometry is described for the laser stripe to enhance control of the transverse mode. In some embodiments, the stripe offset geometry is provided with two parabolic sections coupled to a central straight section.
U.S. Pat. No. 4,942,585 discloses a semiconductor laser having a pumped trapezoidal-shaped gain medium layer between a wide output facet and a narrower mirror facet. The laser provides high power by having a wide output facet so that the power density at the output facet is low enough to avoid catastrophic optical mirror damage. At the end of the gain layer opposite the output, the gain layer is parallel-edged and index-guided to ensure a single-mode output. At the output end, the gain layer diverges from the parallel-edged portion outward to the output facet. The entire diverging region of the gain layer is pumped to stimulate emission of radiation.
U.S. Pat. No. 4,349,905 discloses a stripe semiconductor laser having an active stripe region with a tapered width. The stripe laser structure has a pair of wide sections that allow a low threshold current density for lasing, a narrower section to preclude oscillation in unwanted modes, and a pair of tapered stripe sections connecting the wide sections to the narrow section. It further discloses a stripe laser structure having a single tapered section connecting a wide section with a narrow section, where the narrow section leads to the output facet. In this structure, the narrow stripe width reduces the minimum image size when a tightly focused beam is required.
U.S. Pat. No. 4,689,797 discloses a semiconductor laser having an active layer with a narrow waveguide section and an amplifier section. The narrow waveguide section provides lateral mode stability while the amplifier section provides a large reservoir of injected carriers required for high power lasing. The laser structure further includes a rear facet with a reflectivity between 90-97% near the amplifier section and a front facet with reflectivity below 10% near the waveguide section. The narrower waveguide section, therefore, leads to the output of the device.
UK Patent Application GB 2317744A discloses an incoherent array of tapered semiconductor lasers suitable for materials processing having a ridge loaded or buried laser structure that is formed on a single chip. This application discloses that the lasers forming the array have a straight region and a tapered region. The sides of the tapered section may be straight or follow a parabolic shape and be substantially parallel at the output end. A laser formed in this manner provides an output that can be focused to a small spot so that a material can be sufficiently heated to cause a chemical change, ablation or burning.
Applicant has noted that known laser designs that permit increased power output do not provide advantages in low thermal resistance, low power density in the laser cavity, and overall electrooptical performance of the device.
Moreover, Applicant has observed that conventional high-power lasers do not permit efficient coupling to a single-mode optical fiber without the need for additional optics.
Applicants have found that a high-power semiconductor laser element can desirably obtain high output power with low thermal resistance and low power density in the laser cavity with appropriate configuration of a guiding section, for example of a ridge. Moreover, Applicants have discovered that a semiconductor laser element having a guiding section with a narrow parallel region, a diverging region, and then a wide parallel region adjacent to the output facet produces these advantageous results as well as permits coupling to a single-mode optical fiber without using additional optics.
According to a first aspect, the present invention has to do with a semiconductor element for emitting single-mode high power laser light, comprising a light-guiding section of length L longitudinally extending between a rear facet and a front facet, said light-guiding section comprising:
a narrow portion adjacent to said rear facet having substantially parallel sides, of width W1 and length L1, for guiding a single mode of propagation,
a diverging portion of length L2, widening from width W1 to a width W3, for expanding adiabatically said single-mode of propagation,
The length L1 is greater than 0.4 L, and the light-guiding section comprises a wide portion adjacent to said front facet having substantially parallel sides of width W3 and length L3 greater than 20 xcexcm, said width W3 being in a range between 5 and 20 xcexcm.
In order to form a laser, the rear facet is high reflection coated and said front facet is low reflection coated.
Applicants have found that such a laser can achieve a high reliability under high output power conditions. This makes the laser particularly adapted for pumping optical amplifiers for submarine use, where reliability is a critical issue in view of the high cost of maintenance.
To form an optical amplifier, both said facets are anti reflection coated.
In one embodiment, the wavelength of said emitted laser light are around 980 nm. In this embodiment, W3 is preferably comprised between 5 and 11 xcexcm; preferably W1 is comprised between 3 and 5 xcexcm.
Advantageously, L3 is at least 0.04 L.
Preferably, L3 is at least 0.1 L.
Advantageously, L1 is lower than 0.8 L.
Advantageously, L2 is greater than 100 xcexcm.
Preferably the diverging portion has straight sides having an angle of divergence lower than 2.5xc2x0.
According to a different embodiment the wavelength of the emitted laser light is around 1480 nm.
Typically, the semiconductor element comprises a plurality of layers in a vertical direction.
According to a preferred embodiment, a ridge is defined on at least one of the upper layers of said plurality of layers, thereby defining said light-guiding section.
The semiconductor element advantageously comprises an active layer having top and bottom surfaces; a core layer over each of the top and bottom surfaces of the active layer having refractive index n, wherein n decreases with distance from the active layer; a cladding layer over each core layer, and an upper thin layer on one of the cladding layers and a substrate layer on the other of the cladding layers.
Typically, a pump electrode is defined over the surface of the ridge. The pump electrode can be defined over the whole surface of said ridge. Alternatively, the pump electrode is T-shaped and has a width W1 over the narrow and diverging portion of the light-guiding section and a width W3 over the wide portion of the light-guiding section. According to a further alternative, the pump electrode is a stripe having width W1.
According to a second aspect, the present invention has to do with a pigtailed semiconductor element for emitting single mode high power laser light, comprising a light-guiding section of length L longitudinally extending between a rear facet and a front facet, the light-guiding section comprising a narrow portion adjacent to said rear facet having substantially parallel sides, of width W1 and length L1, for guiding a single mode of propagation, and a diverging portion of length L2, widening from width W1 to a width W3, for expanding adiabatically the single-mode of propagation.
The optical semiconductor element is coupled to a single mode fiber having a mode field diameter MFD, and the length L1 is greater than 0.4 L. The light-guiding section comprises a wide portion adjacent to the front facet having substantially parallel sides of width W3 and length L3 greater than 20 xcexcm. The width W3 is in a range between 0.6 MFD and 1.4 MFD.
Preferably, the width W3 is in a range between 0.85 MFD and 1.15 MFD.
Preferably, the coupling with said single-mode fiber is a butt-coupling.
According to a third aspect, the present invention has to do with an optical fiber amplifier comprising a rare earth doped optical fiber, a pigtailed semiconductor element as indicated above, for providing a pump emission, and a dichroic coupler suitable for coupling the pump emission to the rare earth doped optical fiber.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.